The present invention relates to an internal combustion engine, and more specifically to an engine system capable of changing a combustion mode in an internal combustion engine between a stratified charge combustion mode and a homogeneous charge combustion mode.
A conventional engine control system (as disclosed in Japanese Patent Provisional (Kokai) Publication No. 62(1987)-110536) employs an electronically controlled throttle valve for electronically controlling a throttle opening to achieve a desired target engine torque determined from operating parameters such as the driver's accelerator input and engine speed. However, this system is unable to control the fuel air ratio adequately.
On the other hand, the technique of in-cylinder direct fuel injection in a spark ignition engine is under development to improve the fuel efficiency and emission performance by selectively using stratified charge combustion and homogeneous charge combustion.
In a low and medium load region, a control system for such a type operates an engine in a stratified combustion mode by injecting fuel directly into a combustion chamber during the compression stroke so as to produce a stratified combustible mixture only around the spark plug. The thus-achieved stratified combustion enables stable combustion with an ultra lean mixture, and hence significant improvement in the fuel efficiency and emission performance of the engine.
In a high load region over a predetermined engine load, the engine is operated in a homogeneous combustion mode to meet a demand for higher output torque. In the homogeneous combustion mode, fuel. is injected during the intake stroke so as to produce a homogeneous air fuel mixture. (In some examples, a fuel injection valve is provided separately in an intake port.)
The control system changes over the combustion control mode between the stratified combustion mode and the homogeneous combustion mode in accordance with one or more engine operating conditions.